1. Field of the Invention
This invention relates to electrostatic spray coating systems, and more particularly to use of a device for collecting free ions in an electrostatic spray coating system.
2. Description of the Prior Art
In electrostatic spray coating systems, the coating material is pumped from a supply to one or more spray guns which spray the coating material onto a part to be coated. The coating material may be either in the form of dry particles conveyed in a fluidized air stream or in the form of liquid atomized by the gun. The spray guns may charge the coating particles by means of a high voltage charging electrode. When the coating particles are sprayed from the front of the gun, they are electrostatically attracted to the part to be coated which is generally electrically grounded and suspended from an overhead conveyer in a spray booth. The spray guns are mounted in the spray booth, either in a stationary position or on a reciprocator other device which allows the gun to be automatically moved in a predetermined path. Once these charged coating particles are deposited onto the part, they adhere there by electrostatic attraction until they are conveyed into an oven where they are cured, or, in the case of powder coating, melted to flow together to form a continuous coating on the product.
Anti-back-ionization (ABI) devices or counter-electrodes have been found to be useful in attracting free ions which would otherwise be attracted to the part. When corona charging the powder, a large number of free ions are also generated. Without an ABI device, the free ions are conveyed with the coating material onto the part. This causes a charge to accumulate on the deposited coating layer until the local electric field strength is great enough to cause ionization from the coating layer. This "back-ionization" can disturb the deposited coating and result in craters and other defects in the cured coating. By using an ABI device, these free ions are collected before they reach the part, and the appearance of the surface finish on the part can be improved.
An example of an ABI device is shown in U.S. patent application Ser. No. 08/959,723 assigned to the assignee of the present invention. This patent shows an is ABI probe which provides satisfactory free ion collection. The effective probe length is adjustable, allowing the position of the end of the probe relative to the electrode to be changed, so that the probe position relative to the electrode can be adjusted.
Another example of an ABI device is shown in U.S. Pat. No. 4,921,172, issued to Belmain et al., in the form of a counter-electrode mounted on a powder spray gun on the front of the gun. Yet another example of an ABI device is shown in European Patent Publication No. 0,620,045 in the form of a counter-electrode ring fixedly mounted around the front of the gun. These ABI devices are fixed in position or built into the gun, so that they do not provide easy adjustment or removablity.
A benefit of the ABI probe is that it allows users to reduce dramatically the field strength between the gun and the grounded part as well as to eliminate most of the free ion current to the part. Therefore, development of back ionization is greatly reduced and penetration of Faraday cage areas is greatly facilitated. As a starting point, the ABI probe should be positioned such that the distance between the tip of the gun and the ABI probe is approximately half the distance between the gun and the part. Thus, the same ease of coating of recessed areas can be observed with corona guns as is achieved in tribo applications.
One shortcoming of the ABI probe is that the distance between the gun and the part changes based on the geometry of the part and on changes in the parts assortment. As this distance changes, the positioning of the ABI probe behind the top of the gun must be manually adjusted to provide for maximum positive effect of using the ABI probe as an ion collector. Unfortunately, such manual repositioning of the ABI probe can be rather cumbersome and, as a result, tends to be rarely done in production applications.